Manually operated differential pressure based flow instrument calibration manifolds are common in flow measurement systems. A special type of manifold is unique to the gas flow measurement industry.
Over a decade ago a research arm of the gas industry identified a flow measurement issue unique to compressible gas flow measurement. The issue is called “Gauge Line Error” (GLE). Past practice in the industry was to mount the measurement instruments on pipes or racks away from the primary differential pressure (DP) instrument mounted in the flow line for convenience. Typically long and small diameter pressure lines were run for this purpose. The research entity discovered that when the pressure lines were of small diameter, changed size over the run, or were sufficiently long, the pressure signal would often be distorted through amplification or attenuation of the pressure signal as the line changed diameter or direction, or simply due to the length of the signal line. This would lead to an erroneous DP signal being presented to the DP measurement instrument and thus the flow calculation was erroneous.
The recommendation to address the problem was for the signal lines to be as short as possible and be of constant 0.375 diameter, which is the approximate size of the standard signal line hole in the primary instrument mounted on the pipeline.
Prior to this point in time, the gas flow measurement industry had used a standard sized (0.186 Dia or 0.250 Dia.) soft conical seated valve for the calibration instruments. With the news of the GLE issue, a new type of manifold came into use which was called a “large bore manifold.” The large bore manifold also had a soft cone seat as preferred by the industry, but due to the 0.375 size of the signal bore the hand operated stem of the manual valve had to travel much further than the smaller cone design. This new travel requirement combined with the industry standard 20 to 24 pitch threads on the stem resulted in from approximately 10 to 12 turns of the handle being required to open or close the valve.
Given that an instrument calibration technician can often be called on to calibrate dozens of flow measurement points in a single shift and that each of these requires operating two of these large bore valves in each manifold, reducing the time and number of turns to open and close these valves would be very advantageous both from a time and possible repetitive motion injury basis.
Simply reducing the pitch of the stem thread has its practical limitations, since the stem diameter cannot typically exceed 7/16 inch due to size constraints imposed by the standard bonnet design. To put a 14 pitch thread on a 7/16 diameter stem is approaching the practical limit for that stem diameter due to the thread height of approximately 0.071, whereas the 20 pitch thread typically used for that stem diameter is 0.049.
It would be desirous to reduce the pitch well beyond 14 pitch to minimize the turns required to open and close the valve. At some point, however, the “fast” pitch of the thread would make the valve hard to operate at the typical maximum operating pressures of from 900 to 2,000 psi. Further, at some point the faster pitch stem would allow the valve stem to “self open” under pressure or prolonged vibration that may occur in these applications.
The present invention achieves the goal of minimizing the turns to open and close while preserving the operability and reliability of the valves used in the gas flow calibration manifolds.